Class Acanthodii

The acanthodians — commonly known as the "spiny sharks" — are the earliest-known vertebrates with jaws. These structures are presumed to have evolved from the first gill arch of some ancestral jawless fish which had a gill skeleton made of pieces of jointed cartilage (see p. 20).

The popular name "spiny sharks" is really a misnomer for these early jawed fishes. The name was coined because they were generally shark-shaped, with a streamlined body, paired fins and a strongly upturned tail; stout bony spines supported all the fins except the tail — hence, "spiny sharks."

In fact, acanthodians were a much earlier group of fishes than sharks. They evolved in the sea at the beginning of the Silurian period, some 50 million years before the first sharks appeared (see pp. 26—29). Later the acanthodians colonized freshwaters, and thrived in the rivers and lakes during the Devonian and in the coal swamps of the Carboniferous. But the first bony fishes were already showing their potential to dominate the waters of the world, and their competition proved too much for the spiny sharks, which died out in the Permian.

Many paleontologists consider that the acanthodians were close to the ancestors of the bony fishes. Although their internal skeletons were made of cartilage, a bonelike material had developed in the skins of these fishes, in the form of closely fitting scales. Some scales were greatly enlarged, and formed a bony covering on top of the head and over the lower shoulder girdle. Others formed a bony flap over the gill openings (the operculum in later bony fishes).

name: Climatius time: Late Silurian to Early

Devonian locality: Europe (UK) and North

America (Canada) size: 3 in/7.5 cm long The name "spiny shark" seems particularly appropriate for this fish. Its tiny body was crowded with spines and fins. Two large dorsal fins rose from the back, each supported in front by a stout bony spine, superficially embedded in the skin. There was a large anal fin and spine at the back, and a pair of large pectoral fins with spines at the front.

The underside of Climatius' body bristled with spines, but no fins. There was a pair of pelvic spines and 4 pairs of belly spines.

Climatius was obviously an active swimmer, to judge by its stabilizing fins and the powerful, sharklike tail, with its large, upturned upper lobe. Like many other acanthodians, it had no teeth in its upper jaw, but there were whorls of small teeth in the lower jaw, continually replaced as they were being worn — another sharklike feature. Its large eyes suggest that sight was the chief sense used for locating prey, and it probably fed on crustaceans and fish fry in mid-water and at the surface.

Its swimming agility and the tight-fitting armor of bony scales must have protected Climatius from attack by larger fish and predatory invertebrates, such as squid. The 15 fin spines that arrayed its body were its chief defense, making it extremely awkward to swallow.

name: Acanthodes time: Early Carboniferous to Early

Permian locality: Australia (Victoria),

Europe (Czechoslovakia, England, Germany, Scotland and Spain) and North America (Illinois, Kansas, Pennsylvania and West Virginia) size: 1 ft/30 cm long Acanthodes was a member of the last group of spiny sharks to evolve. They had no teeth in their jaws, but the gills were equipped with long bony "rakers" made of toothlike spikes. Acanthodes and its relatives were probably filter-feeders, sieving tiny, planktonic animals through their gills.

Like all later acanthodians, Acanthodes was larger than its earlier relatives; some members of its group reached lengths of over 6 ft 6in/2m. Acanthodes was also less spiny than earlier forms. Its paired pectoral fins still had stout spines, as did the large anal fin. But there was only one spiny dorsal fin, set far back near the tail, and the pair of the ribbonlike pelvic fins that ran along the belly each had a single spine. Thus, Acanthodes only had 6 fin spines on its body, compared with the 15 spines of its prickly relative, Climatius (above).